In general magnetic disk drives, positional information used to position a magnetic head is written in a servo area as servo information on a magnetic disk medium. Data information is written to areas, called sectors, between servo information.
The magnetic head senses servo information at certain time intervals, specifies the position of its track and/or its position in a radial direction while performing a sequence of positioning operations such as “following” and/or “seek”.
The magnetic head comprises a combination type magnetic head in which an MR head using a magnetoresitive effect in reproduction and a thin film head using a thin film technique for coil production in record are mounted on corresponding sliders, respectively.
In data renewal, the magnetic disk drive generally uses a method of directly writing new data over old-written data without erasing the old data, which is referred to as “overwrite”.
A. When data is to be renewed, it is important to accurately position the magnetic head at the center of the track. To position the magnetic head less accurately causes the respective positions of the already written data and new data to be written to deviate from their proper positions. In this case, a part of the old data, which is not renewed or updated by the new data, could remain present and be possibly decoded.
Most of positioning errors of the magnetic head which would occur due to various causes can be eliminated by the servo control circuit. However, with vibrations at a frequency higher than a sensing cycle of the servo information, the prior-art positioning errors can not be reduced. Therefore, it is very difficult to completely eliminate the remaining old data.
In order to solve this problem, JP-A-6-338010 has proposed a method of preventing old data decoding, by positioning a head at a plurality of different positions on a magnetic disk medium when data is to be decoded, and checking to see whether or not respective data to be decoded coincide.
In this method, 1) since the head is positioned at a plurality of different positions on the magnetic disk medium and the data are compared, the magnetic disk should be rotated at least twice and data reproduction time increases; and 2) this method is not effective for an abnormal operation and failure of the writing head.
B. With the complex magnetic head, there are the problems of an abnormal operation and a failure of the writing head. Old writing heads each comprise a ring-like magnetic core of high permeability and a coil wound around the core in which a current is caused to flow through the coil to produce a magnetic field, which writes information to a magnetic disk medium.
The causes of malfunctions in the writing head are 1) disconnection and short-circuits in the coils/current paths through which the currents flow, 2) corrosion of the writing head itself, physical/mechanical breakage of the writing head due to contact with the magnetic disk medium, and a deterioration in the performance of the magnetic core; and 3) an increase in the spacing between the writing head and the magnetic disk medium due to the depositing of foreign substances on the surface of the magnetic head facing the medium. These events greatly reduce the writing magnetic field, so that it is impossible to erase the old data completely by renewing the old data with new data or by writing new data over the old data.
In order to sense the state in which old data remains due to abnormality of the writing head, a history of data renewal in the location where the old data remains must be managed. To this end, it could be considered to use, for example, a method of mounting a semiconductor memory on the magnetic disk drive to thereby leave history information. According to this method, both the positioning errors on the writing head and abnormality of the writing head are eliminated. Writing and recording information to and from the semiconductor memory is performed at very high speeds and the processing time hardly increases.
JP-A-5-66999 discloses storing and managing management information on a cache management table of a RAM.
When data renewal information in all the sectors are to be stored in a semiconductor memory, however, the capacity of the semiconductor memory needs to increase, which runs counter to cost reduction. Since a capacity of the semiconductor memory necessary to store the data renewal information is about 1/1000 of the whole data quantity and if it is considered that the cost of the semiconductor memory is several hundreds times high per unit storage capacity compared to the cost of the magnetic writing medium, the cost of the whole magnetic disk drive would increase about scores of percents.